The present invention relates in general to sewing machines and in particular to a stop motion device for stopping the motion of a sewing, embroidering or tufting machine, when one of a plurality of threads being supplied to the machine breaks.
A stop motion device of this kind is known from U.S. Pat. No. 2,664,840. In this device there is provided for each thread a sensing lever pivoting about a horizontal axis and having two arms of different weight, the lighter, thread-sensing arm being arranged between a thread tensioning device and a thread deflecting guide. Due to the unequal weight of the arms and the suspension of the sensing levers on a horizontal axis/axle, the sensing levers are pretensioned by gravity against the threads to be monitored for rupture. The sensing levers are held in their sensing position by the tensile stress acting on the threads due to the pulling of an additional thread from the thread supply through the thread tensioning device to make up the thread comsumption during stitch formation. In case of thread rupture, the respective sensing lever swings out about the bearing axis due to the higher dead weight of the one arm, and in so doing it actuates a switching lever which is common to both sensing levers, to stop the machine.
In other known stop motion devices which, however, are not suitable for monitoring several threads, the sensing lever is pretensioned against the supplied thread by spring or magnetic force and is held, by the tensile stress acting on the thread when the machine is running, in a sensing position from which, in case of a thread rupture, it is moved by the spring or magnetic force into the position for stopping the machine (German utility model No. 19 14 424; U.S. Pat. No. 3,714,916).
In these known stop motion devices the thread is stressed transversely of its feed direction. In double chain stitch and overcast sewing machines, the transverse stress on the thread or threads has an extremely unfavorable effect on the looping process, i.e. on the forming of a thread chain for example between workpieces successively fed at intervals, because the tensile stress acting on the thread is very much smaller in the looping process than in the forming of a seam in the work, so that the sensing lever acting on the thread with pretension crosswise to the feed direction may increase the tensile stress and thereby adversely affect the looping process. Besides, with each stopping of the machine, that is, when the pull on the thread ceases, the sensing lever is urged by its dead weight or the spring or magnetic force into a position causing the stopping of the machine. When the machine is restarted, these stop motions require certain circuit measures to overcome the interruption of the motor circuit caused by the sensing lever when the machine is turned on again.
Lastly, the sensing lever acting on the thread may cause the needle thread end to be pulled out of the eye of the needle after the cutting of the thread. To prevent this, the needle thread end must be clamped in a thread clamp.